FIG. 1 shows an exploded view of a conventional module 10 which is used for housing a Liquid Crystal Display (LCD) 14. The module 10 includes a front frame 12, an LCD 14, a thin film resistive heater 16, and a back frame 18. The device 10 is held together by interlocking tabs and recesses.
As is known in the art, if the temperature of the LCD 14 becomes too cold, the liquid crystal material within the LCD 14 becomes increasingly viscous. If such a result occurs, the LCD 14 does not work properly.
In view of this problem, the thin film resistive heater 16 is provided to maintain the temperature of the LCD 14 within a certain temperature range. To perform its heating function, the thin film resistive heater 16 includes a glass substrate 20 which contains a thin film coating of, for example, indium tin oxide (ITO). The substrate 20 is attached to a plastic frame 22. As is known in the art, when a current is passed through the thin film coating, heat is transferred to the glass substrate 20 thus creating a heater.
When the module 10 is assembled, glass substrate 20 of the thin film heater 16 is placed in thermal contact with the LCD 14. This configuration allows for the transfer of heat from the heater 16 to the LCD 14.
Typically, a thermistor 24 is placed on an outer edge of the glass substrate 20 to monitor the temperature of the thin film heater 16. As is known in the art, the thermistor is a device whose resistance is a function of temperature.
Based on the design of the device shown in FIG. 1, the thermistor 24 is required to be placed on an outer edge of the thin film heater 16 to ensure that the thermistor 24 does not interrupt the viewing area of the LCD 14. In particular, if the thermistor 24 is positioned at the center of the glass substrate 20, a shadow will appear on the LCD 14. This is obviously undesirable given that such a shadow would impact the usefulness and desirability of the LCD 14.
FIG. 2 shows a conventional circuit that is used to control and measure the temperature of the thin film heater 16 shown in FIG. 1. The circuit shown in FIG. 2 contains a power supply 26 which provides a supply voltage V.sub.SUPP to bus bars 21 of the glass substrate 20 contained within the thin film heater 16. As is known in the art, the application of the voltage V.sub.SUPP creates a current I which in turn heats the substrate 20.
The circuit shown in FIG. 2 also contains a temperature feedback circuit 28 which is connected to the power supply 26 and the thermistor 24. The temperature feed back circuit 28 continuously measures the temperature of the thermistor 24.
If the temperature of the thermistor 24 falls below a certain temperature To , the feedback circuit 28 instructs the power supply 26 to apply the voltage V.sub.SUPP to the bus bars 21 to heat the glass substrate 20. Conversely, if the temperature of the thermistor 24 reaches temperature To, the feedback circuit 24 then instructs the power supply 26 to remove the voltage V.sub.SUPP.
While the conventional device discussed above and shown in FIGS. 1 and 2 allows for the temperature of the thermistor 24 to be measured, the device still has significant drawbacks. In particular, the design of the device shown in FIGS. 1 and 2 results in a large thermal mass at the edge of the thin film heater 16 where the thermistor 24 is located. As is known in the art, large thermal masses resist changes in temperature. That is, large thermal masses either maintain temperature for prolonged periods of time or require an inordinately large amount of heat to achieve an increase in temperature.
Given that the edge of the thin film heater 16 is in contact with a large thermal mass and that the thermistor 24 is required to be positioned at this location, it is difficult to accurately monitor and maintain the temperature of thin film heater 16. In view of this problem, there currently exists a need for a device which can accurately measure the temperature of the center of a thin film heater in a manner that is minimally impacted by the thermal mass proximate to the heater and does not affect, in any way, the viewing area of the heater.